With a liquid chromatograph, a gas chromatograph and the like, by irradiating measurement light on a sample inside a cell, for example, and by detecting transmitted light from the sample by a detector, chromatogram data representing the relationship between time and intensity (absorbance or the like) may be obtained. If a PDA detector (Photodiode Array Detector) is used as the detector, light from the sample dispersed by a diffraction grating or the like may be detected by a plurality of photodiodes for respective wavelengths, and thus three-dimensional data representing the relationship between time, wavelength and intensity may be obtained.
When an impurity analysis is performed for a pharmaceutical product by using this type of chromatograph, the proportion of impurities to a main component in a sample is sometimes analyzed. In this case, when the concentration difference of components in the sample is great, there are problems that, if a low-concentration component (impurity) is to be accurately detected, the signal of a high-concentration component (main component) is saturated, and if the high-concentration component (main component) is to be accurately detected, the low-concentration component (impurity) is buried in the noise.
For example, it is described in “Acetylcysteine, Purity (6) Related Substances” (pp. 311-312) in Non-Patent Document 1 that, if a test is conducted by a liquid chromatograph that uses an ultraviolet absorption photometer at a measurement wavelength of 220 nm, the areas of peaks other than acetylcysteine are each not more than 0.3%, and the total area is not more than 0.6%.